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The GA standard is a 1/4" plug for the speakers, and a 0.206" plug for the microphone. The latter stems from the telephone switchboards of the early 20th century,

Actually, they both stem from that source, Ron. The Bell System was still using both at least into the 1970s. And not just the plug/jack, but also the electrical characteristics (impedance, etc.) were also telephone standards, to the point that I was actually able to take an early electronic telephone headset and easily adapt it to aircraft usage, mostly by changing the plugs.

I think phone chargers and electric razors have different plugs and voltages there, but again Im not sure.

Many countries use 240 VAC where the U.S. used 120 VAC, and many countries run that AC power at 50 Hz (cycles per second) vs. 60 Hz in the U.S., so yes, those items are different, but they have nothing to do with what aircraft use.

So if your helmet has the ¼" audio plug, and the .206 mic plug, it should work fine in their aircraft that are so equipped. But if your helmet has the military style 4-conductor plug (I think it's similar to many helicopters here) then it'll only work in aircraft so equipped.

I'll own up to being a Brit on the forum!
Headsets generally no problem. The radios are mostly Garmin, King ... the same as everywhere else.
The laws of physics are quite similar so not much change in the mechanics of flying.

What you may find different is:

No publicly funded airfields, so you will be expected to pay for the privilege of using them.
You usually enter the circuit (pattern) from the overhead crossing the upwind end of the runway at 90 degrees and turn on to the downwind. No 45 joins.
VFR advisories are called a 'Basic service' and the service is sporadic and a squawk code is limited to one controller. Conspicuity is 7000 not 1200.
Many places will require you to wear a high-visibility vest while air side.
Most small airfields have an air-ground frequency which has an operator who can give information but not instructions. Make calls to the station like you would on CTAF; downwind, finals to land etc which will be replied with roger/traffic info/wind info etc.
Non radio airfields use a system similar to CTAF on 135.480 MHz to be used to self announce when close to the field. Radios are now 8.33 KHz channel spacing (2280 channels).
Airways are class A down to quite low levels, so VFR is generally forced to fly at the lower levels.
It rains a lot and 100LL is expensive.

Thanks, Mark. Are their common British parachute makers or do they mostly use the standard American ones like Strong, Butler, Softie etc.?

There are British parachute manufacturers - IrvinGQ mostly supply the forces, but do sell to individuals too. A few others supply to the skydiving and sailplane communities as well as the aerobatic with seat pack and back pack types. The US ones you mention are also fairly commonly used.
The skydiving clubs are usually happy to re-pack both British and US types for a reasonable cost.